Due to a recent spate of terrorist activity, security has been stepped up at airports, bus and train stations, shopping centers and malls, and at museums, landmarks and other potential targets.
Baggage is often subjected to X-Ray examination; however because of the health risks involved with exposure to radiation, such techniques are not favoured for examination of persons.
Where necessary, frisking or strip searching of persons may be performed, but such procedures are very time-consuming, and the subjects of such examinations may consider these procedures embarrassing and degrading.
For detection of chemical substances such as drugs and plastic explosives, dogs have been found effective. For detecting weapons such as knives and guns, metal detectors are widely used.
Two common metal detectors in widespread use are the wand-type metal detector and the frame-type metal detector. The principle of operation of both these devices is similar, in that they both detect the presence of metal objects using eddy-currents and the electromagnetic effect. The wand-type metal detector is quickly swiped over and around the abdomen and perhaps the thighs of the subject Although minimally intrusive and not particularly pleasant, it has the advantage over conventional frisking in that it keeps the hands of the operator distanced from the body of the subject. Indeed, if used skillfully, the wand-type detector need not actually touch the subject at all. If a metal object, such as a door key or loose change, is found, the subject is invited to remove and display the object himself and the examiner need neither touch the possessions nor the body of the subject.
The frame-type metal detector, see for example U.S. Pat. No. 3,971,983 to Jaquet, resembles a door frame through which the subject is required to pass. Indication of a metal object by a beep or a light, typically results in the subject being required to remove metal objects from about his person, and put them in a tray that is passed around the frame.
Neither of the above metal detector types is recommended for use with subjects fitted with pacemakers, where interference with the pacemaker may be injurious to health. Another problem with their use is that neither is effective for disclosing metal objects concealed within the shoes of the subject, or strapped around the ankle or lower leg. The wand-type detector is not conveniently swiped over these lower extremities, and indeed cannot be done without compromising the security of the personnel operating the device. The frame-type detector has low sensitivity near the ground, by virtue of the ground tending to provide an alternative, preferred pathway for electromagnetic field lines.
The poor performance of metal detector systems of the prior art with regard to detection of objects concealed on the lower leg or foot, makes these locations a likely concealment area for knives or hand guns for hijackers, terrorists and smugglers.
One simple idea for overcoming this problem is described in U.S. Pat. No. 5,039,981 to Rodriguez, which discloses a riser platform, that is, a step, for introducing into the detector frame. The subject is caused to pass through the electromagnetic field at a height above ground level, and may be requested to stand on the step. Being largely free from ground and other forms of interference, an enhanced sensitivity is achieved.
Metal detecting security equipment is typically positioned near entrances to buildings, and there are various sources of electromagnetic interference, such as the proximity of automatic doors, electronic machinery, people using mobile phones and the like, which generate stray electromagnetic fields that can act as sources of noise, triggering false alarms. Static electricity charges picked up from artificial carpets, and conveyer belts and escalators may cause similar problems. Shoe heels are frequently attached to the shoe with nails, and some shoes contain eyelets for laces, buckles, steel toe caps, strike plates and/or steel heel inserts. Thus, in addition to being excellent places of concealment for weapons, they may also trip a metal detector and generate false alarms. However subjects may find being asked to remove their shoes humiliating. Shoe removal is also time-consuming, difficult for the arthritic, and unpleasant for all concerned where the subject suffers from foot-odour, for example.
There is thus a need for a metal detecting system that has enhanced sensitivity for weapons concealed in the shoe or on the lower leg, yet having a reduced susceptibility to false alarms from metal objects legitimately found in shoes, and the present invention is directed to providing that need.